Known pneumatic systems for vehicles, in particular for commercial vehicles, operate with a compressor that, controlled by an electronic control device of a compressed air supply system, compresses air drawn in from the atmosphere. The control device switches the compressor on for supply phases if compressed air is required in the pneumatic system, and off for idle phases when no further compressed air supply is necessary. The supplied compressed air is provided to different devices of the vehicle as required, such as brake systems and auxiliary units. For dehumidifying the compressed air, as a rule an air drying device with a replaceable moisture absorbing cartridge is provided. In order to remove the water collected there, from time to time in a so-called regeneration process dry air from the pneumatic system is passed in the opposite direction through the air drying device and is discharged into the surroundings by means of a venting valve. At low temperatures, however, freezing of the precipitated water can occur in the region of the air drying device, in particular at the venting valve. As a result, malfunctions in the pneumatic system can be caused.
In order to prevent icing of the pneumatic system at low temperatures or to melt an already existing ice formation, a controllable heating element can be disposed on the air drying device, which is energized as required and prevents ice formation by giving off heat. However, such a heating element increases the costs of manufacture of the pneumatic system and the heater operation increases the energy consumption of the vehicle. It is also known to switch over the pneumatic system into a so-called cold mode by means of the control device for compressed air supply to protect against icing at low ambient temperatures. In the cold mode the compressor is switched to continuous operation by means of a simple control function, at least for a fixed predetermined period of time. The compressed air that is heated by the compression process in the compressor gives off heat to the air drying device during this, so that the components at risk of freezing do not ice up. However, additional compressed air is produced by the temporary continuous operation in the cold mode that is not always required by the pneumatic system. The excess compressed air can be discharged to the surroundings by opening the venting valve. By feeding in warm compressed air and discharging warm compressed air, the system components, in particular the venting valve, are also kept working at low temperatures. However, the temporary additional continuous operation of the compressor increases the wear thereof and the energy consumption of the pneumatic system.
An air drying device of a pneumatic system for vehicles and a method for preventing ice formation on a venting valve device of such an air drying device are generally known. For example, the air drying device may comprise a compressor connector for connecting a compressor, a venting connector for connecting the air drying device to the atmosphere by means of the selectively opened or closed venting valve device and a working connector of the pneumatic system. With the venting valve device closed, compressed air supplied by the compressor to the compressor connector is conveyed into the pneumatic system by means of the working connector. With the venting valve device opened, compressed air supplied by the compressor to the compressor connector is discharged into the atmosphere, wherein the compressed air heated in the compressor is fed to the venting valve device by a direct flow path without any detours. The warm compressed air is supplied to the venting valve device as required, in particular at low temperatures, as a preventative measure to protect against icing.